I used to do a lot of competitive trail rides. Even sponsored and put on a ride for 4 years. So for what it worth here is my advice.
As stated above. Who cares what the horse looks like?
I knew a man who rode CTR ever other weekend he was 86 years old and his horse was 28 years old. They rode in the Open Class and often won. So don't worry about a 15 year old horse.
You need to be more concerned about the horses conditioning. Do you know how to take your horses Pulse and Respiration? If not learn how and as your condition the horse watch his P&R's.
CTR has several classes of riders. You will start out in the Novice Class. Which is a shorter distance at a slower speed with easier obsticles. Your combined two day ride will be about 40 miles total. ( Open riders do about 50 miles) So the question is can your horse do a 20 mile ride at 4-5 mph? That is a brisk walk with a few trots for a novice rider. So take your horse out and do some practice rides. Maybe start out with 10 mile rides, Walk briskly, Maybe 5 miles into the ride stop and give him a 10 minute rest and then immediately take his P&R's After 10 minutes of rest he probably should have pulse and respiration of less than 12 beats or breaths in a 15 second period. If his P&R's are higher than 12 after a 10 minute rest, He is being stressed and you need to maybe cut the ride short. Work up to where he can do 5 miles, then 10 miles, then 10 miles with the last mile at a trot, then 15 miles with a couple of trots some where in that distance. When your horse can cover 20 miles and not show signs of stress, you are ready for a CTR.
As the distances increase, you will also need to pay attention to hydration in addition to P&R rates. Start doing Pinch or tent test on his shoulder, Look at the color capillary refill of his gums. Become familair with his normal gut sounds and be able to hear if his gut sound dimish. during rides give him every chance to drink. If it's a long ride ( like 5-6 8 hours) give him a chance to graze somewhere along the trail. Pull up under a tree and enjoy the shade for a few minutes and let him enjoy the green grass. You want to keep a distance horse's gut working all day. Also take a few minutes along the ride to try various obsticles. You won't always be moving down the trail during a CTR. Stop and give the horse a break and ask him to step over a log, sidepass over and hang your coat on a branch, Open a gate from the saddle, something to ask for control vs just mortoring down the trail.
After the ride look at his legs, Both immediately , several hours after and even the next day. Look and see if he has any filling in his pasterns. Filling is not as much of a problem with younger horses, But older horse will show the swelling. Especially if he is out of shape. Part of CTR is the conditioning of the horse. You can fairly quickly build up the soft tissues (heart and lungs), The harder the tissue the longer it takes to condition it. So ligiments and tendons take longer to build than lungs and muscles. Bones take even longer. So as you start to work him harder you need to watch for signs of stress in these harder tissue. You want enough work to strengthen the tissue but not cause lameness.
Check his back, several hours after you finish the days ride. See if he has any sore spots along his spine. Even if his saddle fits, some horses get excited during a ride, they tense up and/or won't collect up. This can cause sore muscles along the spine or on the croup.
These are all things you will learn as you do CTR. So don't think you need to master all of this before doing a CTR. But just start to notice and pay attention. It will help you to know when your horse is conditioned enough. Most horses a 20 mile ride is not a big deal. But if he is fat and out of shape, it will stress him.
Good luck and have fun.